This invention relates generally to a method of measuring the momentum transfer spectrum of elastically scattered X-ray quanta by means of which it can be concluded on which material the X-ray quantum was elastically scattered.
An apparatus for the examination of items of luggage is described in EP 1 241 470 B1. Such an apparatus has a focus extended in Y-direction which emits X-radiation in the X-direction. Through a primary collimator extending in the Y-direction only X-ray quanta which are aimed at an individual isocentre are allowed through into an examination area lying behind the isocentre. The isocentre forms the originating point of a Cartesian coordinates system and a disk-shaped inverted fan beam is thus formed. A scatter collimator which is developed annularly about the Z-direction is arranged between the isocentre and the examination area. The result is that only scattered radiation from an object located in the examination which emanates from the scatter voxel at a fixed preset angle of scatter is allowed through. A detector which extends along the Z-axis is arranged in the Y-Z plane. Depth information regarding the scatter voxel, i.e. its X-coordinate, is thereby imaged onto a parallel of the Y-axis in the Y-Z plane. By means of such an arrangement a rapid analysis of an item of luggage can be achieved, with only a one-dimensional movement of the item of luggage along the Z-direction on a conveyor belt having to take place. The scanning speed is, however, limited by the angle-dependent sensitivity of the detector elements.
Attempts have been made to replace the detectors arranged merely in Z-direction with a detector array, located in the Y-Z plane, consisting of a series of detector elements. Due to geometric imaging conditions the scatter quanta of a scatter voxel are always imaged onto elliptical structures in the Y-Z plane. By integrating the scatter quanta in the associated detector elements a greater yield and thus a faster recording of the material at the location of the scatter voxel takes place. Alternatively, a better signal-to-noise ratio is obtained for the momentum transfer spectrum. It has been found that with such apparatus only those methods belonging to a single fixed, preset angle of scatter can be carried out. However, depending on the material density in the item of luggage to be examined, the energy of the photon, and thus of the angle of scatter, can be varied while the momentum transfer function remains unchanged. Accordingly, a compromise is to be found for the examined angle of scatter.